In recent years, capacitive touch panels are commonly used as input devices for electronics. Projecting type capacitive touch panels include two electrodes for detecting a change in capacitance. The two electrodes are opposed to each other with a transparent substrate interposed therebetween. Those electrodes are formed by patterning a transparent conductive film deposited on the substrate.
A typical material for the transparent conductive film deposited on the substrate as an electrode is indium tin oxide (ITO). A method for depositing ITO on the substrate is vacuum deposition in a dry method. However, a main component of ITO, indium, is a rare metal and a stable supply cannot be secured. Further, there is another problem that indium lacks flexibility. In addition to that, manufacturing of ITO needs an expensive vacuum deposition machine, leading to an increase in manufacturing cost.
In light of the above problems, alternative materials to ITO have emerged. Specifically, conductive films are formed by using conductive polymers, carbon nanotubes, metals formed in a fibrous shape or a mesh shape. Some of those materials can be dispersed in water or an organic solvent. Those dispersed liquids can be applied on the surface of the base material in a wet method. This also allows for mass production and cost reduction. In particular, the conductive film formed on the base material by using fibrous or mesh shaped metal seems to be promising as an alternative to ITO in that it exhibits a resistance and optical properties similar to those of ITO.
Although the touch panel having a conductive film formed of fibrous metals operates properly, it has a problem of lack of durability when used in the environment of high temperature and high humidity. Under an environment of high temperature and high humidity, malfunctions such as erroneous recognition of touch position and drop of capacitance can occur. It seems that those malfunctions occur due to migration of fibrous metals that form the electrodes. Migration is a phenomenon in which, when a voltage is applied to the electrodes under high temperature and high humidity, metal in the electrodes is ionized due to the presence of water, and the metal ions move from anode to cathode. Those ions receive electrons at the cathode, thereby allowing the metal to be deposited and grow on the surface of the insulator in the form of dendrites, bridges, clouds or the like. When the deposit reaches the anode, short-circuiting occurs, resulting in malfunctions. The base material used as the touch panel includes a plurality of electrodes in the shape of bars or diamonds which are arranged side by side. The adjacent electrodes are insulated by etching. In order to drive the base material as the touch panel, routed wirings are provided on each of the electrodes so that the electrodes are supplied with voltage from an IC circuit via a flexible printed circuit board (FPC). As the voltage is applied to each of the electrodes, migration of fibrous metals that form the electrodes occurs due to the effect of water under high temperature between the adjacent electrodes having potential difference. This causes a decrease in capacitance and short-circuit of electrodes, and as a result, the touch panel fails to operate properly.
In order to solve the problem, there is a technique of blocking water to prevent migration. For example, as described in PTL 1, a water blocking layer is provided to prevent water infiltration and thus prevent occurrence of migration. However, increase of the layers may cause problems such as increase in the touch panel thickness, increase in the amount of material and increase in the number of processes, which may lead to increase in costs.
Furthermore, a drive electrode and a sense electrode which form a sensor unit of capacitive touch panels are formed of a transparent conductive film (transparent electrode) and is generally connected to a metal wiring (wiring section). Such a touch panel is described in PTL 2. In the touch panel described in PTL 2, a dummy lead wiring is provided on each end of a lead wiring, and the dummy lead wiring is connected so as to be at a predetermined potential of a detection wiring which is not selected.
In general, in this type of touch panel, the wiring section or the sensor section is shielded from the effect of outside noise by covering the wiring section from the upper or lower side or providing a ground electrode outside the wiring section. When the ground electrode is provided in the same layer as the drive electrode, it is disposed so as to overlap the wiring of the sense electrode or disposed in the peripheral area. Further, when the ground electrode is provided in the same layer as the sense electrode, it is disposed so as to overlap the wiring of the drive electrode or disposed in the peripheral area.
In such a case, since an electric field of a specific direction is generated between the ground electrode and the drive electrode or the sense electrode, a problem has been raised depending on the material of the transparent conductive film used for the drive electrode or the sense electrode, in which the transparent conductive film becomes broken due to ion migration, or short-circuits between the transparent conductive film and the ground electrode. In particular, ion migration occurring under high temperature and high humidity has been a matter of concern.